In Act 1, when you first get access to the abbey, check the Prior’s House (in the Old Bailey) and inspect the volvelle.
Once you start working in the scriptorium and the other monks leave, Ferenc will come and write something in his book.
Once he’s gone and you’ve helped sister Illuminata find her books, inspect Ferenc’s book.
After the next time skip (at Nones), you can explore again. Go to speak to Werner outside his house in north Tessing. He’ll explain the link between astrological signs and the elements. I think if you have certain backgrounds for Andreas you might be able to skip this step.
Go back to the Prior’s House and interact with the volvelle. Now you can solve the cypher.
How to solve the puzzle:
*** Spoiler - click to reveal ***The four colours on the outer wheel relate to the four elements. For each letter on the note:
- match the astrological symbol on the note with one of the four elements
- turn the wheel to match the element colour with the large triangle at the top of the volvelle
- select the corresponding Greek letter from the note on the body of the volvelle (in gold) - the game does this automatically if you move the cursor left and right along the note
If you're really stuck:
*** Spoiler - click to reveal ***Sequence of elements/colours to match with the arrow at the top:
- yellow
- green
- red
- blue
- red
- yellow
- green
- green
- blue
It should spell ‘higerhard’
Once you've solved it, keep going through the dialogue options. Andreas will eventually work out what the message means and the achievement should pop.